Field of the Invention
Embodiments of the present invention are related to a control circuit which controls an output voltage, and a switching power supply.
The present application claims priorities on Japanese Patent Application No. 2015-027366, filed Feb. 16, 2015, Japanese Patent Application No. 2015-029525, filed Feb. 18, 2015, Japanese Patent Application No. 2015-029954, filed Feb. 18, 2015, and Japanese Patent Application No. 2015-169595, filed Aug. 28, 2015, the contents of which are incorporated herein by reference.
Description of Related Art
As a method of controlling a switching power supply, hysteresis control using a hysteresis comparator is known (see JPH03-293965A and JP2014-57476A). In the hysteresis control, a reference voltage which is input to a non-inverting input of a comparator takes a first high voltage when the output of the comparator takes a high level (H level) and a second low voltage when the output of the comparator takes a low level (L level). The difference between the first voltage and the second voltage is a hysteresis width. When a voltage in which an output voltage is resistance-divided is brought to be lower than the reference voltage of the second voltage, the output of the comparator is brought to be at the high level (H level) to start a drive period, causing the reference voltage to be brought to the first voltage. When a current which is larger than a load current is supplied from the switching power supply to an output capacitor to raise the output voltage during the drive period to bring the voltage in which the output voltage is resistance-divided to be higher than the reference voltage of the first voltage, the output of the comparator is brought to the low level (L level) to complete the drive period, transitioning to a stop period. At this time, the reference voltage is brought to be the second voltage. In the stop period, no current is supplied from the switching power supply and a load current is supplied from the output capacitor, so that the output voltage decreases. JP2014-05476A discloses that a phase-compensating capacitor C2 which is connected in parallel with a resistance which voltage-divides an output voltage is further added to make more stable control possible.
Moreover, burst control in which switching is performed a plurality of times during a period in which a hysteresis comparator takes a high level (H level) is known as a control method in which the hysteresis comparator is used. JP2007-181389A discloses that a hysteresis comparator which compares an output voltage and a reference voltage and a circuit which turns off a gate voltage during a period when a current of a switch element reaches a value are provided. As a process is repeated in which, during the period in which the hysteresis comparator is at the high level (H level), the gate voltage is turned on to cause the current to reach the value and turn off the gate voltage, the gate voltage is again turned on as the period in which the hysteresis comparator is at the high level (H level) continues, and the current again reaches the value to turn off the gate voltage, switching is performed a plurality of times during the period in which the hysteresis comparator is at the high level (H level). When the hysteresis comparator is at the low level (L level), the gate voltage does not turn on, so that switching stops. During the period in which the hysteresis comparator is at the high level (H level), switching is repeated, so that supplying a current which is larger than a load current from the switching power supply to the output capacitor raises the output voltage. During the period in which the hysteresis comparator is at the low level (L level), switching is stopped, so that no current is supplied from the switching power supply and the load current is supplied from the output capacitor, decreasing the output voltage.
Problems of the above-described control methods include that, when the output voltage is quite high compared to the reference voltage and a voltage-dividing ratio when the output voltage is divided by resistance dividing to around the reference voltage is large, an output voltage ripple is brought to the product of the voltage-dividing ratio and the hysteresis width, which is the difference between the first voltage and the second voltage. To prevent erroneous operation due to noise, the hysteresis width may not be decreased unlimitedly. Moreover, in the phase-compensating capacitor C2, which is disclosed in JP2014-057576A, it is difficult to control output voltage fluctuations to the hysteresis width. It is desirable for the output voltage ripple to fall within the hysteresis width.
Moreover, a soft start function for slowly raising the output voltage for preventing an inrush current at the time of activating the switching power supply is known.
For example, WO2005/101629A discloses that a capacitor voltage for soft start is instantaneously charged to the same level as a triangular wave for PWM from a beginning of activation to shorten the period from the beginning of the activation to a beginning of a rise of an output voltage.
Moreover, JP2013-240159A discloses that a soft start voltage is increased rapidly from a beginning of activation to detecting a PWM pulse signal to shorten a period from the beginning of the activation to a beginning of a rise of the output voltage.
Problems of the above-described control methods include that, when a capacitor with a large static capacitance is used for the control circuit of the switching power supply, it takes time for a voltage of the capacitor to be charged from 0V at the time of activation to a steady state value, so that it takes time for the output voltage to reach a target value at the time of activation.